piston choice

[mb911]

Ok I am going to be looking for pistons for my 3.o within the next 6 months and want to gather some info.

first of all my engine will be mildly boosted .4 bar etc. It will be based off an 81 sc bottom end with sc cams probably.

what would suggest and why

3.0 78-79 pstons and cylinders nikisil?
80-83 pistons and cylinders Nikisil/aluisil? (and why)
930 3.3 pistons and cylinder(why and what would be involved in this any machining etc)
3.2 Pistons and cylinders nikisil or aluisil (and why)

What mods to the cae would I need to do. I am thinking the low 8's to middle 8's for Cr would be best what do you all think??

[Jeff Alton]

If you need to buy a P/C set, how about bored and plated 98's and JE pistons at 8ish:1 CR. Then you would have a SS3.2 turbo!!

For your turbo motor I would think you want NEW pistons. I realize $$ can be an issue on any rebuild (don't know your situation) but if you have some laying around maybe even consider some cylinders from Charles.

The 3.3 turbo pistons would have a different pin diameter than your 3.0 ones so that may require a crank and rods. Same goes for the 3.2 pistons.

So, if you are set on getting used, I would look for some low CR SC piston and cyliners.

In one of your other post I saw mention of shimming the cylinders to reduce the CR. While this will lower the CR and increase the deck height, increased deck height can contribute to increased detonation.

Cheers

I

[mb911]

I have not determined what exactly I am going to do. I guess I want to understand fully what my options are. also why changing deck height will that make more likely to detonate??

I was also under the impression that 3.2 pistons were a "bolt on" fpr my application?

[cgarr]

As long as your in there and got the heads off think about twinplug?

[mb911]

Originally posted by cgarr
As long as your in there and got the heads off think about twinplug?

Craig. I thought about it whats it cost??

[Henry Schmidt]

It just so happens we have some turbo pistons with 22 mm wrist pins.
Almost new and good used.

CIS injections tend to run lean so if you're thinking about 8.0:1 + you will need to control the ignition to prevent detonation. Some kind of enrichment devise is also advised.

If you find a good set of 3.3 pistons and cylinders you can convert the rods on your 3.0 to suit those pistons. This will deliver a relatively low compression allowing you to run more boost.

[mb911]

Originally posted by Henry Schmidt
It just so happens we have some turbo pistons with 22 mm wrist pins.
Almost new and good used.

CIS injections tend to run lean so if you're thinking about 8.0:1 + you will need to control the ignition to prevent detonation. Some kind of enrichment devise is also advised.

If you find a good set of 3.3 pistons and cylinders you can convert the rods on your 3.0 to suit those pistons. This will deliver a relatively low compression allowing you to run more boost.

How much henry? Also I am running EFi so I can control AFR,s Pretty well. I was running 11.2 -11.5 so I was pretty safe. Maybe now time for edis??

[sww914]

Too much deck height, or too little deck height can cause detonation because you will have the wrong amout of space around the sides of the combustion chamber for the A/F mixture to flow into, changing the shape of the explosion in the chamber, causing parts of the volitile mixture to explode too soon. It will either hit the top of the piston still on it's way up, or essentially cause two explosions to occur, colliding into each other, and either way it's bad for your pistons.

[mb911]

ok so I have not really go a clear answer would all these options be viable? If I found a set of 3.4 turbo pistons with the correct spigots sizing would that work with my crank and rods? if not would I need to modify the bushings in the rods??? What about the 3.2 pistons? Again just looking for good info before I do anything. I need to sell a bunch more mufflers first

[Jeff Alton]

The rod length is different between your rods and the 3.2/turbo rods. The piston pin diameter is also different. So they are not "bolt in" Henry says he can modify SC rods to work with the 23mm pin pistons (turbo, 3.2) I am not sure what happens to the CR when you do this. But... the 3.0 rod does come with larger rod bolts which is a plus.

I guess (just a guess) if the CR ended up to low you could (within reason) machine some of the heads or cylinder tops to raise it up a wee bit. It seems to me Henry knows best on this one though.

For simplicity, I would get your cylinders reconditoined (or look into Henry's QSC cylinders) and use a JE with the correct pin diameter and with the CR you want right out of the box. May not be the most cost effective but you have essentially new parts that fit with no screwing around, which, IMO, is worth a little.

Hope that helps a little!

Cheers

[1fastredsc]

Ben you may want to talk pistons and cylinders in terms of piston size, not engine displacement. When you say 3.2 pistons, it could mean the aftermarket 98mm pistons and cylinders with the short stroke of the SC crank which makes a 3.2. On the other hand you could also be talking about the 3.2 carrera stuff, which stock had a longer stroke with the same size 95mm pistons that the SC's have, but with different dimensions on other parts of the pistons, which even if adapted would still make a 3.0 with the shorter SC crank.

[mb911]

ah thats the info I needed thank you. thats what I thought but needed conformation

[cstreit]

Originally posted by catca
In one of your other post I saw mention of shimming the cylinders to reduce the CR. While this will lower the CR and increase the deck height, increased deck height can contribute to increased detonation.

Jeff,

I don't know anything about turbo's so pardon what may be a dumb question.... Can you explain this too me? On NA engines a lower deck height is a detonation worry, why is a turbo different?

[JeremyD]

Ben - for a turbo engine I would look at the jugs that JB racing sells - they are iron sleeved and they have had good results with sealing and no blowby. It's much more critical that you get a good piston to cylinder seal on a turbo engine. I'm sure you know that. Another source would be the Nickies sold by LNengineering.



Couple that with some JE pistons - and you could get any compression ratio that you need.

[safe]

Hmm, I thought your turbo kit was supposed to be a low boost, cheap and easy bolt on kit

[1fastredsc]

Yeah, did you pop your motor already, or are you just looking to crank up that boost

[mb911]

Originally posted by safe
Hmm, I thought your turbo kit was supposed to be a low boost, cheap and easy bolt on kit

it is was. The signal hose to the Waste gate melted And I did not realize it and over boosted a few to many times. Guess I needed a display boost gauge and AFRs would be nice. We have 3 other guys out there running very successfully with the same setup but better hose set up which we will change to hard line for safety

[Jeff Alton]

Chris, this thread has interesting explanaitons of Deck Height. I am not sure if a turbo suffers like a NA motor does with a raised deck height.

See Grady's post on page 2.

9.8 or 10.1?

Cheers